The present invention relates to purification of exhaust gases of engines.
It is of course not new, and even is currently practised in some automobiles, to mount catalytic converters containing some oxidizing and/or reducing catalyst in the exhaust system of the engine, for the purpose of purifying the exhaust gases of noxious components such as NOx, HC, and CO so that NOx is reduced to innoxious N.sub.2 while HC and CO are oxidized to innoxious CO.sub.2 and H.sub.2 O. As a catalyst used for the purification of exhaust gases, there is known a 3-way catalyst which accomplishes simultaneously both acceleration of the reduction of NOx and acceleration of the oxidization of HC and CO. With regard to the purification of NOx, the 3-way catalyst shows a high NOx purification performance in a reducing gas medium richer than a stoichiometric gas mixture, whereas the NOx purification performance abruptly lowers when exhaust gases are diluted by air beyond the stoichiometric gas mixture and are converted into an oxidizing medium including surplus oxygen. On the other hand, with regard to the performance in purification of HC and CO, the 3-way catalyst obviously shows a high HC and CO purification performance when the exhaust gases constitute an oxidizing gas leaner than the stoichiometric gas mixture, but the HC and CO purification performance abruptly lowers when the exhaust gases constitute a reducing mixture richer than the stoichiometric mixture. Because of this characteristic, in the conventional exhaust gas purification system employing the 3-way catalyst, it has been contemplated to maintain exhaust gases exactly in the stoichiometric condition. Therefore, it has been the general practice to effect a strict air/fuel ratio control for the exhaust gases introduced into the catalytic converter by employing an O.sub.2 sensor or the like. In this case, it has been contemplated that the air/fuel ratio equivalent of exhaust gases is to be maintained within a relatively narrow range such as 14.5.+-.0.2.
However, it is very difficult and requires expensive control means to maintain exhaust gases within such a narrow range around the stoichiometric condition, because the constitution of exhaust gases of an engine changes greatly in accordance with the operating conditions of the automobile.
In view of the aforementioned problems with the conventional exhaust gas purification system for automobiles employing a 3-way catalyst, in a co-pending U.S. patent application Ser. No. 818,870 filed July 25, 1977, now abandoned and assigned to the same assignee as the present application, we have proposed a novel system for purifying exhaust gases of engines which is simple in structure and inexpensive, and yet accomplishes an improved overall exhaust gas purification performance, characterised by a method of purifying exhaust gases of engines comprising the processes of passing the exhaust gases through a 3-way catalyst in a reducing condition richer than the stoichiometric mixture, thereby purifying the exhaust gases of NOx substantially to the target purification rate while simultaneously purifying them of a part of the HC and CO contained therein, adding air to the exhaust gases to convert them into an oxidising medium leaner than the stoichiometric mixture, and then passing the exhaust gases through a second catalyst which accelerates at least the oxidizing reaction thereby purifying the exhaust gases of the remaining HC and CO therein to the final target purification rate.
As mentioned above, a 3-way catalyst has the general characteristic that NOx purification performance abruptly lowers as exhaust gases shift to an oxidizing condition leaner than the stoichiometric mixture while its HC and CO purification performance abruptly lowers as exhaust gases shift to a reducing condition richer than the stoichiometric mixture. In this case, however, it is noted that the falling off rate of HC and CO purification performance in the reducing mixture condition is relatively moderate when compared with the falling off rate of NOx purification performance in the oxidizing condition. The aforementioned method proposed in the co-pending application for purifying exhaust gases of engines originates from noticing this particular characteristic of a 3-way catalyst and emerges from the conventional art of confining the operational region of a 3-way catalyst within a narrow band of the stoichiometric exhaust gas mixture having an air/fuel ratio equivalent such as 14.5 (stoichiometric).+-.0.2, in a manner such that the operational region is positively shifted to a reducing region richer than the stoichiometric mixture, thereby avoiding the critical region where NOx purification performance abruptly lowers, and that a 3 -way catalyst is used principally for purifying the exhaust gases from NOx under a relatively inexact control of the exhaust gas condition with an incidental purification from a part of HC and CO. In this case, exhaust gases are processed by the modified application of a 3-way catalyst in a manner such that NOx is removed substantially to the final target purification rate while a part of the HC and CO are incidentally removed, and then the exhaust gases are supplied with a supply of air and are converted into an oxidizing mixture leaner than the stoichiometric mixture, and then the exhaust gases are passed through a second catalyst which accelerates at least the oxidizing reaction wherein the HC and CO remaining in the exhaust gases are further removed to the final target purification rate.
In practicing the aforementioned method of purifying exhaust gases, the air/fuel ratio equivalent of the exhaust gases entering into a 3-way catalyst is controlled in a range of approximately 13.5-14.6 in view of the general purification performance of a 3-way catalyst. The air/fuel ratio equivalent of the exhaust gases entering into the second catalyst should preferably be controlled within a range of approximately 14.5-18.0. With regard to the purification rate of the three components NOx, HC and CO, in view of the general purification caracteristics of a 3-way catalyst, it is desirable that the 3-way catalyst purifies the exhaust gases from NOx up to approximately 80-90% while incidentally purifying them from HC up to approximately 80-90% and from CO up to approximately 50-80%, and that the second catalyst purifies them from HC to the fianl target purification rate such as approximately 90-98% and from CO to the final target purification rate such as approximately 85-98%.
By employing the concept of controlling the air/fuel ratio equivalent of exhaust gases within a relatively wide range such as approximately 13.5-14.6 for operating a 3-way catalyst by contrast to the conventional concept of controlling the air/fuel ratio equivalent of exhaust gases within a very narrow range such as 14.5.+-.0.2, it is contemplated that the air/fuel ratio control for exhaust gases is exempt from the conventional strict feedback control depending upon an O.sub.2 sensor for detecting oxygen density in the exhaust gases.